Detergent compositions used in laundering textile items such as clothing or fabrics typically contain a surfactant as a detergent component. Further, a variety of other additives are also added for the purpose of imparting various functions, including detergent builders such as alkaline agents, enzymes, hydrotropic agents, preservatives, antibacterial agents, fluorescent brightening agents, colorants, fragrances and antioxidants.
Among the various additives added to detergent compositions, enzymes are an important functional material, and are used as an additive that is capable of producing excellent detergency even under the severe laundering conditions (such as low temperature, low concentration, and short washing times) used in Japan.
Furthermore, in recent years, due to increased awareness of environmental issues, and also for design reasons, water-conserving drum-type washing machines are becoming increasingly popular for household washing machines. However, it has become evident that when laundering is performed using a typical household drum-type washing machine, because the liquor ratio (the ratio of the weight of wash liquid relative to the weight of items being laundered) is small, resoiling of the fibers tends to occur easily.
In order to enhance the anti-resoiling properties of liquid detergent compositions, a method that uses a specific nonionic surfactant (Patent Document 1), and a method that combines a specific nonionic surfactant, an anionic surfactant and an enzyme (Patent Document 2) and the like have been proposed.
On the other hand, in recent years, products have appeared in which enzymes have also been added to liquid detergent compositions, which are becoming increasingly popular as clothing detergents. However, the stability of enzymes in liquid detergent compositions is inferior to that of powdered detergent compositions, and a problem arises in that the enzyme activity is lost over time, making it impossible to achieve a satisfactory effect. This stability tends to be particularly problematic when the enzyme is used in combination with an anionic surfactant.
Numerous techniques for stabilizing enzymes added to liquid detergent compositions have been investigated, and methods that have been proposed include a method in which free calcium ions are added (Patent Document 3), a method in which a short-chain carboxylate such as a formate or a lactate is added (Patent Document 4), a method in which a specific polyol and boric acid or a derivative thereof are added (Patent Document 5), a method in which the ratio between a nonionic surfactant and an anionic surfactant, the pH, the degree of alkalinity and the water content and the like are set within specific ranges (Patent Document 6), and a method in which a (meth)acrylic acid/(meth)acrylic acid copolymer and a polyethylene glycol are added (Patent Document 7).
Besides these stabilization techniques, methods in which a reducing agent is added to improve the enzyme activity have also been proposed. For example, Patent Document 8 discloses a method in which an organic compound (such as an organic reducing agent) that cleaves disulfide bonds is added to a surfactant and a protease.